Many different methods for the preparation of epoxides have been developed. One such method involves the liquid phase epoxidation of an olefin with an organic hydroperoxide in the presence of a solubilized transition metal catalyst. Although highly active and selective for olefin epoxidation, soluble catalysts must be recovered or recycled after use to avoid loss to a waste stream. However, it can be very expensive to recover the soluble catalysts after use. In addition, recycle decreases catalyst productivity by also recycling certain heavy substances such as acids and polymers that tend to accumulate along with catalyst in the heavy bottoms stream. The recycled heavies' stream decreases epoxide selectivity or olefin conversion.
Heterogeneous (insoluble) catalysts have been developed to avoid homogeneous catalyst disadvantages. U.S. Pat. No. 4,367,342 discloses an olefin epoxidation process in the presence of an insoluble catalyst comprised of an inorganic oxygen compound of titanium. Unfortunately, the disclosed catalysts have less than optimum activity and selectivity. British Pat. No. 1,332,527 teaches a process for preparing an improved titania-silica catalyst characterized by impregnating an inorganic siliceous solid with a titanium compound in an oxygen-substituted hydrocarbon solvent, removing the solvent, and calcining the impregnated solid. Suitable solvents are limited to oxa and/or oxo-substituted hydrocarbons that are liquid at ambient conditions including alcohols, ketones, ethers, and esters. According to this patent, impregnation in an oxygen-substituted hydrocarbon solvent produced catalysts with improved properties compared to similar catalysts prepared by other methods. The alleged reason is that such catalysts have a more uniform, non-agglomerated content of titanium dioxide.
A later-filed patent application (EP 345,856) discloses the preparation of epoxidation catalysts that are alleged to be more active than the analogous catalysts obtained by previously known procedures. EP 345,856 teaches impregnation of silica with a gaseous stream of titanium tetrachloride, followed by calcination, hydrolysis, and, optionally, silylation. In a comparative example, a catalyst prepared by silica impregnated with a solution of tetra isopropyl ortho-titanate, complexed with acetyl acetone in isopropanol solvent, was found to be 4.5 times less active than the catalyst prepared by vapor phase impregnation with titanium tetrachloride. Additionally, PCT Int. Appl. WO 98/50374 discloses a catalyst prepared by a liquid phase impregnation process with a non-oxygen containing solvent. The catalyst prepared by this method has activity similar to that produced by the method of EP 345,856. Although WO 98/50374 discloses that higher surface area siliceous solids can incorporate more titanium, it does not disclose any benefits with higher surface area solids.
New methods to produce heterogeneous catalysts for olefin epoxidation have focussed on the use of high surface area, mesoporous supports such as MCM-41 and MCM-48. The methods include direct synthesis in which titanium is incorporated into the framework of the support (see Tanev, et. al., Nature (1994) V. 368, 321) and a grafting technique in which titanocene dichloride is grafted onto a mesoporous silica (see Maschmeyer, et. al., Nature (1995) V. 378, 159). Titanocene dichloride is taught to be superior to titanium tetrachloride or titanium isopropoxide due to lesser tendency to form unwanted oligomeric titanium-oxo species.
We have discovered an effective, convenient method of producing catalyst compositions having high epoxidation activity (and selectivity). These new catalyst compositions are significantly more active than catalysts obtained by techniques taught in EP 345,856, WO 98/50374, or by Maschmeyer, et. al.